Low-melting-point PE film and preparation method thereof

Abstract

The invention relates to the technical field of PE films, and particularly discloses a low-melting-point PE film and a preparation method thereof.The low-melting-point PE film comprises an outer layer, a middle layer and an inner heat sealing layer which are sequentially stacked, and the outer layer is prepared from, by mass, 30%-70% of low-density polyethylene, 20%-50% of linear low-density polyethylene, 5%-10% of linear low-density polyethylene, 5%-10% of linear low-density polyethylene, 5%-10% of linear low-density polyethylene, 5%-10% of linear low-density and 10%-30% of an elastomer. The middle layer is prepared from the following components in percentage by mass: 10 to 50 percent of low-density polyethylene, 10 to 50 percent of elastomer, 5 to 30 percent of ethylene-vinyl acetate copolymer and 1 to 10 percent of plasticizer; the inner heat sealing layer is composed of, by mass, 60%-90% of low-density polyethylene and 10%-40% of elastomer. Through the design of the inner heat sealing layer of LDPE / POE and the synergistic effect of the EVA function of the middle layer, the melting temperature (90-110 DEG C) of the film is remarkably reduced, the heat sealing strength (larger than or equal to 5 N / 15 mm) is improved, and the low-temperature heat sealing film is suitable for the fields needing low-temperature heat sealing or melting such as food packaging, medical instrument packaging and industrial packaging (feeding bags).

Description

Technical Field

[0001] The present invention relates to the technical field of PE films, and particularly relates to a low-melting-point PE film and a preparation method thereof. Background Art

[0002] Traditional PE (polyethylene) films have a relatively high heat-sealing temperature (usually ≥ 110°C) and a high melting point, which easily causes heat damage to heat-sensitive products (such as frozen foods and medicines) during the encapsulation process. Existing low-melting-point films mostly use LDPE and LLDPE materials, which have defects such as high heat-sealing temperature and difficulty in melting. In addition, for multi-layer composite films, it is difficult to meet the low-melting requirements due to the higher melting point of the surface layer material. Therefore, there is an urgent need for a PE film that can achieve low-temperature heat-sealing and melting by optimizing the material ratio and co-extrusion process.

[0003] The existing technology still needs to be improved and developed. Summary of the Invention

[0004] In view of the above deficiencies of the existing technology, the purpose of the present invention is to provide a low-melting-point PE film and a preparation method thereof, aiming to solve the problem that it is difficult to achieve low-temperature heat-sealing and maintain appropriate mechanical strength for existing PE films due to the relatively high melting point of the surface layer material.

[0005] The technical solution of the present invention is as follows:

[0006] A low-melting-point PE film, wherein the low-melting-point PE film comprises an outer layer, an intermediate layer, and an inner heat-sealing layer which are sequentially stacked. Among them, the outer layer is composed of 30%-70% of low-density polyethylene, 20%-50% of linear low-density polyethylene, and 10%-30% of elastomer by mass percentage; the intermediate layer is composed of 10%-50% of low-density polyethylene, 10%-50% of elastomer, 5%-30% of ethylene-vinyl acetate copolymer, and 1%-10% of plasticizer by mass percentage; the inner heat-sealing layer is composed of 60%-90% of low-density polyethylene and 10%-40% of elastomer by mass percentage.

[0007] For the low-melting-point PE film described above, the density of the low-density polyethylene is 0.915 - 0.925 g / cm 3 , and the melt index is 4.0 - 7.0 g / 10 min.

[0008] For the low-melting-point PE film described above, the density of the linear low-density polyethylene is 0.915 - 0.923 g / cm 3 , and the melt index is 1.0 - 2.0 g / 10 min.

[0009] For the low-melting-point PE film described above, the density of the elastomer is 0.88 - 0.905 g / cm 3, with a melt index of 1.0 - 2.0 g / 10 min.

[0010] The low - melting - point PE film described above, wherein the content of vinyl acetate in the ethylene - vinyl acetate copolymer is 12 - 18%.

[0011] The low - melting - point PE film described above, wherein the plasticizer is epoxy soybean oil or dioctyl phthalate.

[0012] A method for preparing the low - melting - point PE film as described in the present invention, which includes the steps:

[0013] Mix the raw materials of the outer layer, the middle layer, and the inner heat - seal layer in proportion for standby;

[0014] Add the raw materials of each layer into three extruders respectively for melting and plasticizing. The extrusion temperatures are set as follows: for the outer layer, 160 - 190 °C; for the middle layer, 160 - 180 °C; for the inner heat - seal layer, 150 - 180 °C. Extrude and form through a three - layer co - extrusion die head, and shape and set by air cooling or water cooling to form a three - layer co - extruded film;

[0015] Trim and slit the three - layer co - extruded film according to the size requirements, and wind and roll it up to obtain the low - melting - point PE film.

[0016] The method for preparing the low - melting - point PE film, wherein the flow channel of the three - layer co - extrusion die head is designed as a spiral superposition type, and the thickness ratio of each layer is outer layer: middle layer: inner heat - seal layer = 3:4:3.

[0017] Beneficial effects: Through the synergistic effect of the inner heat - seal layer design of LDPE / POE and the function of the middle - layer EVA in the present invention, the melting temperature of the film is significantly reduced (90 - 110 °C) and the heat - seal strength is improved (≥5 N / 15 mm). It is applicable to fields such as food packaging, medical device encapsulation, and industrial packaging (feeding bags) that require low - temperature heat - sealing or melting. Brief Description of the Drawings

[0018] Figure 1 It is a schematic structural diagram of a low - melting - point PE film of the present invention. Detailed Embodiments

[0019] The present invention provides a low - melting - point PE film and a method for preparing the same. To make the purpose, technical solution and effects of the present invention clearer and more definite, the present invention is further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0020] Please refer to Figure 1 , Figure 1 It is a schematic structural diagram of a low - melting - point PE film provided by the present invention,

[0021] As shown in the figure, the low melting point PE film comprises an outer layer, an intermediate layer and an inner heat-sealing layer which are sequentially stacked. Among them, the outer layer is composed of 30%-70% low density polyethylene, 20%-50% linear low density polyethylene, and 10%-30% elastomer by mass percentage; the intermediate layer is composed of 10%-50% low density polyethylene, 10%-50% elastomer, 5%-30% ethylene-vinyl acetate copolymer, and 1%-10% plasticizer by mass percentage; the inner heat-sealing layer is composed of 60%-90% low density polyethylene and 10%-40% elastomer by mass percentage.

[0022] Through the design of the inner heat-sealing layer of LDPE / POE and the synergistic effect of the EVA function of the intermediate layer in the present invention, the melting temperature of the film is significantly reduced (90-110 °C) and the heat-sealing strength is improved (≥5 N / 15 mm), which is applicable to fields such as food packaging, medical device encapsulation, and industrial packaging (feeding bags) that require low-temperature heat-sealing or melting.

[0023] Specifically, the outer layer is composed of 30%-70% low density polyethylene (LDPE), 20%-50% linear low density polyethylene (LLDPE), and 10%-30% elastomer (POE). Among them, the density of the low density polyethylene is 0.915-0.925 g / cm 3 , and the melt index is 4.0-7.0 g / 10 min; the density of the linear low density polyethylene is 0.915-0.923 g / cm 3 , and the melt index is 1.0-2.0 g / 10 min; the density of the elastomer is 0.88-0.905 g / cm 3 , and the melt index is 1.0-2.0 g / 10 min. In this composition, the molecular chain of LDPE contains more short branches, and the intermolecular force is relatively weak, which enables it to soften and melt at a lower temperature, helping to reduce the overall melting temperature of the film. The molecular chain of LLDPE has a higher regularity and a relatively large crystallinity. After being blended with LDPE, while ensuring a certain flexibility, it improves the tensile strength and rigidity of the film, thus providing a stable support structure for heat-sealing. The long-chain structure and good elasticity of POE play a toughening role in the blending system, can effectively disperse stress, prevent rupture caused by stress concentration during the heat-sealing process, and ensure the reliability of heat-sealing.

[0024] In this embodiment, the intermediate layer is composed of 10%-50% low density polyethylene, 10%-50% elastomer, 5%-30% ethylene-vinyl acetate copolymer, and 1%-10% plasticizer by mass percentage. Similarly, the density of the low density polyethylene is 0.915-0.925 g / cm 3, with a melt index of 4.0 - 7.0 g / 10 min; the density of the elastomer is 0.88 - 0.905 g / cm 3 , with a melt index of 1.0 - 2.0 g / 10 min; the content of vinyl acetate in the ethylene-vinyl acetate copolymer is 12 - 18%; the plasticizer is epoxy soybean oil or dioctyl phthalate. In this composition, the synergistic effect of LDPE and POE provides flexibility and certain strength to the middle layer, ensuring the stability of the film during processing and use. The VA content in EVA is 12% - 18%, and the polar groups of VA disrupt the crystalline structure of polyethylene, reducing the crystallinity of the polymer, thus lowering the melting temperature of the film. The plasticizer molecules insert between the polymer molecular chains, weakening the intermolecular forces, making the molecular chains slide more easily, further reducing the melting temperature of the film, while improving the flexibility and plasticity of the film, which helps the layers to better fuse during heat sealing and enhances the heat sealing effect.

[0025] In this embodiment, the inner heat-sealing layer is composed of 60% - 90% low-density polyethylene and 10% - 40% elastomer by mass percentage. Among them, the density of the low-density polyethylene is 0.915 - 0.925 g / cm 3 , with a melt index of 4.0 - 7.0 g / 10 min; the density of the elastomer is 0.88 - 0.905 g / cm 3 , with a melt index of 1.0 - 2.0 g / 10 min. The low melting point characteristic of LDPE enables the inner heat-sealing layer to melt at a lower temperature, realizing the heat-sealing operation; the addition of POE, with its good elasticity and compatibility, can better fuse with LDPE during the heat-sealing process, increasing the flexibility and cohesion of the heat-sealing area, effectively improving the heat-sealing strength, enabling the film to form a firm sealing structure after heat-sealing, and preventing problems such as leakage.

[0026] In this embodiment, the outer layer provides heat-sealing support, the middle layer reduces the melting temperature and enhances flexibility, and the inner heat-sealing layer realizes low-temperature melting and improves heat-sealing strength. The components of each layer cooperate with each other, and finally a low-melting-point PE film with low melting and good heat-sealing strength is manufactured.

[0027] In some embodiments, a method for preparing a low-melting-point PE film is also provided, which includes the steps of: mixing the raw materials of the outer layer, middle layer, and inner heat-sealing layer in proportion for standby; adding the raw materials of each layer into three extruders for melting and plasticizing respectively, and setting the extrusion temperature as: 160 - 190 °C for the outer layer, 160 - 180 °C for the middle layer, and 150 - 180 °C for the inner heat-sealing layer, extruding and forming through a three-layer co-extrusion die head, and sizing and shaping by air cooling or water cooling to form a three-layer co-extruded film; trimming and cutting the three-layer co-extruded film according to size requirements, and forming and winding to obtain the low-melting-point PE film.

[0028] Specifically, in this embodiment, the raw materials of the outer layer, the middle layer, and the inner heat-sealing layer are mixed in proportion for standby. According to different usage requirements and performance needs, the proportion of the raw materials of each layer can be flexibly adjusted, so as to prepare low-melting-point PE films with different performance characteristics to meet the requirements of various application scenarios. Three extruders are used to melt and plasticize the raw materials of each layer respectively, and are extruded and formed through a three-layer coextrusion die head. This multi-layer coextrusion method can complete the preparation of multi-layer films at one time, with high production efficiency. At the same time, it can also ensure the tight combination between layers and improve the overall performance of the film. The extrusion temperature of each layer is accurately set: 160 - 190 °C for the outer layer, 160 - 180 °C for the middle layer, and 150 - 180 °C for the inner heat-sealing layer. Precise temperature control helps the raw materials to be melted and plasticized under the best temperature conditions, ensuring the quality and performance of the film. The different temperature settings are determined according to the characteristics of the raw materials of each layer and the structural requirements of the film, which helps to achieve the specific functions of each layer, such as the strength of the outer layer, the toughness of the middle layer, and the good heat-sealing performance of the inner heat-sealing layer. It is shaped and cooled by air cooling or water cooling, providing two cooling methods for selection, which can be optimized according to the actual production situation and the specific requirements of the film. The air-cooling method may be more suitable for the situation where higher surface quality of the film is required and slow cooling is needed to avoid stress generation. The water-cooling method has a fast cooling speed and can improve production efficiency, and is suitable for some occasions with higher requirements for production speed. The three-layer coextruded film is trimmed and slit according to size requirements, formed and wound up, which can accurately control the size of the film, meet the requirements of different users for film specifications, improve the accuracy and consistency of the product, and reduce material waste.

[0029] In some embodiments, the flow channel of the three-layer coextrusion die head is designed as a spiral superposition type, and the thickness ratio of each layer is outer layer: middle layer: inner heat-sealing layer = 3:4:3.

[0030] The spiral flow channel designed in this embodiment can make the material continuously change its direction and speed during the flow process, increasing the shearing and mixing effects between materials. For raw materials with different properties, this flow channel design can make them mix more evenly, so that the final film is more stable and uniform in performance. The spiral superposition method enables the materials of each layer to fit together better during the extrusion process. When the material flows in the spiral flow channel, the interface between layers is continuously updated and contacted, which is beneficial to improving the interlayer adhesion force, forming a good combination between the three-layer films, and improving the overall strength and stability of the film, and it is not easy to appear delamination phenomena. The spiral flow channel can also make the flow of the material in the die head more stable, reducing flow dead corners and pressure fluctuations, which helps to ensure the stability of the extrusion process, make the thickness of the film more uniform, the surface quality better, and reduce film defects caused by unstable extrusion, such as uneven thickness and rough surface.

[0031] The present invention will be further explained and illustrated through specific embodiments as follows:

[0032] Example 1

[0033] A low-melting-point PE film, wherein the low-melting-point PE film comprises an outer layer, an intermediate layer, and an inner heat-sealing layer which are stacked in sequence. Among them, the outer layer is composed of 60% low-density polyethylene, 30% linear low-density polyethylene, and 10% elastomer by mass percentage; the intermediate layer is composed of 40% low-density polyethylene, 40% elastomer, 18% ethylene-vinyl acetate copolymer, and 2% plasticizer by mass percentage; the inner heat-sealing layer is composed of 75% low-density polyethylene and 25% elastomer by mass percentage.

[0034] The preparation steps of the low-melting-point PE film include:

[0035] Mix the raw materials of the outer layer, the intermediate layer, and the inner heat-sealing layer in proportion for standby;

[0036] Add the raw materials of each layer into three extruders respectively for melting and plasticizing. The extrusion temperature is set as follows: 180°C for the outer layer, 170°C for the middle layer, and 160°C for the inner heat-sealing layer. Extrude and form through a three-layer co-extrusion die head, and be shaped by air cooling or water cooling to form a three-layer co-extruded film. The flow channel of the three-layer co-extrusion die head is designed as a spiral superposition type, and the thickness ratio of each layer is outer layer:intermediate layer:inner heat-sealing layer = 3:4:3;

[0037] Trim and slit the three-layer co-extruded film according to the size requirements, and form and wind it up to obtain the low-melting-point PE film.

[0038] Example 2

[0039] A low-melting-point PE film, the formulations of the outer layer and the intermediate layer of the low-melting-point PE film are the same as those in Example 1, the difference is that the formulation of the inner heat-sealing layer is modified to: the inner heat-sealing layer is composed of 65% low-density polyethylene and 35% elastomer by mass percentage. The preparation method is the same as that in Example 1.

[0040] Comparative Example 1

[0041] A low-melting-point PE film, the formulations of the outer layer and the intermediate layer of the low-melting-point PE film are the same as those in Example 1, the difference is that POE in the formulation of the inner heat-sealing layer is replaced by LLDPE, that is, the inner heat-sealing layer is composed of 75% low-density polyethylene and 25% linear low-density polyethylene by mass percentage; the preparation method is the same as that in Example 1.

[0042] Comparative Example 2

[0043] A low-melting-point PE film, the formulations of the outer layer and the inner encapsulation layer of the low-melting-point PE film are the same as those in Example 1, except that no plasticizer is added to the intermediate layer, that is, the intermediate layer is composed of 40% low-density polyethylene, 40% elastomer, and 20% ethylene-vinyl acetate copolymer by mass percentage.

[0044] The low-melting-point PE films prepared in Examples 1-2 and Comparative Examples 1-2 were tested for heat-sealing temperature and heat-sealing strength. Among them, the heat-sealing strength was tested according to ASTM F88 standard; the heat-sealing temperature can be determined by measuring the heat change (endothermic / exothermic) of the material during heating to determine its melting temperature range. DSC can accurately measure the melting peaks of each component in the PE film (such as LDPE, EVA, etc.) and infer the starting heat-sealing temperature. The measured results are shown in Table 1.

[0045] Table 1 Test Results

[0046]

[0047]

[0048] It can be seen from the results in Table 1 that in this example, through the appropriate combination of polymer materials and specific processes, a low-melting-point PE film with low melting and good heat-sealing strength is manufactured. Specifically, from the data comparison between Example 1 and Comparative Example 1, the heat-sealing temperature in Comparative Example 1 increased significantly (125°C vs. 96°C in Example 1), which indicates that the absence of POE led to an increase in the crystallinity of the inner layer and an increase in the melting temperature; although the linear structure of LLDPE enhanced the rigidity, it sacrificed the low-temperature meltability; the heat-sealing strength in Comparative Example 1 decreased (5.5 N / 15 mm vs. 6.8 N / 15 mm), indicating that POE improved the melting interface combination through the viscoelastic behavior of the elastomer, and its absence led to an increase in the brittleness of the sealing interface and a decrease in strength; the above comparison results show that the role of POE in the inner layer is not only to lower the melting point, but also to significantly improve the heat-sealing strength by improving the compatibility and elasticity of the blend system.

[0049] From the data comparison between Example 1 and Comparative Example 2, the heat-sealing strength in Comparative Example 2 decreased (6.0 N / 15 mm vs. 6.8 N / 15 mm), indicating that the plasticizer (such as DOP) improved the fluidity and viscosity of the intermediate layer by weakening the intermolecular force between polymer chains and promoted the layer bonding; its absence led to insufficient bonding force at the melting interface. The change in the heat-sealing temperature in Comparative Example 2 was small (98°C vs. 96°C), indicating that EVA and POE in the middle layer could still lower the melting point, but the absence of the plasticizer limited the optimization of the melting fluidity. The above comparison results show that: the plasticizer is crucial for improving the heat-sealing strength by improving the processing performance and interfacial adhesion in the intermediate layer.

[0050] From the data comparison between Example 1 and Example 2, since the POE content in the inner seal layer of Example 2 increased to 35%, its heat-sealing temperature further decreased (93°C vs. 96°C), indicating that increasing the POE content to a certain extent will further disrupt the crystalline structure of LDPE and reduce the overall melting point; however, the heat-sealing strength in Example 2 increased significantly (8.2 N / 15 mm vs. 6.8 N / 15 mm), indicating that a higher POE content enhances the elasticity and cohesion of the molten interface, forming a denser and more flexible sealing layer. The above comparison results show that optimizing the POE content can simultaneously reduce the melting point and improve the heat-sealing strength, but its proportion needs to balance the mechanical properties (such as tear resistance).

[0051] In summary, the core role of the elastomer (POE) is to reduce the crystallinity, disrupt the ordered structure of LDPE, and significantly reduce the melting temperature; through the viscoelastic properties of the elastomer, the flexibility and bonding strength of the heat-sealing interface are improved. The function of the plasticizer is to weaken the intermolecular force, enhance the fluidity and viscosity of the intermediate layer, strengthen the interlayer bonding force, and synergistically with EVA further reduce the melting temperature. The necessity of the layered design of the low-melting-point PE film in the present invention lies in: the outer layer (LLDPE / LDPE / POE) provides mechanical support; the intermediate layer (EVA / plasticizer / POE) serves as a functional layer to reduce the melting point and promote adhesion; the inner heat-sealing layer (high POE content) directly optimizes the heat-sealing performance. The experiments of the comparative examples verified that the absence of both POE and the plasticizer led to performance deterioration, proving their irreplaceability in the formulation, and at the same time highlighting the importance of the layered collaborative design for achieving the balance between low melting point and high heat-sealing strength.

[0052] It should be understood that the application of the present invention is not limited to the above examples. For those of ordinary skill in the art, improvements or transformations can be made according to the above description, and all such improvements and transformations should fall within the protection scope of the appended claims of the present invention.

Claims

1. A low-melting-point PE film, characterized in that, The low melting point PE film includes an outer layer, an intermediate layer, and an inner heat-sealing layer which are stacked in sequence. Among them, the outer layer is composed of 30%-70% low-density polyethylene, 20%-50% linear low-density polyethylene, and 10%-30% elastomer by mass percentage; the intermediate layer is composed of 10%-50% low-density polyethylene, 10%-50% elastomer, 5%-30% ethylene-vinyl acetate copolymer, and 1%-10% plasticizer by mass percentage; the inner heat-sealing layer is composed of 30%-70% low-density polyethylene, 10%-50% linear low-density polyethylene, and 10%-40% elastomer by mass percentage.

2. The low-melting-point PE film according to claim 1, wherein The density of the low-density polyethylene is 0.915 - 0.925 g / cm 3 , and the melt index is 4.0 - 7.0 g / 10 min.

3. The low-melting-point PE film according to claim 1, characterized in that, The density of the linear low density polyethylene is 0.915 - 0.923 g / cm 3 , and the melt index is 1.0 - 2.0 g / 10 min.

4. The low-melting-point PE film according to claim 1, characterized in that, The density of the elastomer is 0.88 - 0.905 g / cm 3 , and the melt index is 1.0 - 2.0 g / 10 min.

5. The low melting point PE film according to claim 1, wherein The content of vinyl acetate in the ethylene-vinyl acetate copolymer is 12-18%.

6. The low melting point PE film according to claim 1, characterized in that, The plasticizer is epoxy soybean oil or dioctyl phthalate.

7. A method for preparing a low-melting-point PE film according to any one of claims 1-6, characterized in that, It includes the steps: Mix the raw materials of the outer layer, the intermediate layer, and the inner heat-sealing layer in proportion and set aside; Add the raw materials of each layer into three extruders respectively for melting and plasticizing. The extrusion temperature is set as follows: 160-190°C for the outer layer, 160-180°C for the middle layer, and 150-180°C for the inner heat-sealing layer. Extrude and form through a three-layer co-extrusion die head, and be shaped by air cooling or water cooling to form a three-layer co-extruded film; Trim and slit the three-layer co-extruded film according to the size requirements, and form and wind it up to obtain the low melting point PE film.

8. The preparation method of the low melting point PE film according to claim 7, characterized in that, The flow channel of the three-layer co-extrusion die head is designed as a spiral superposition type, and the thickness ratio of each layer is outer layer: intermediate layer: inner heat-sealing layer = 3:4:3.

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